Light-metal molded bodies can be produced as solid bodies, as hollow bodies and as metal-foam bodies. In the first case, the molten metal can be cast in a mold and is intended as much as possible to have a uniform structure. In the casting of hollow bodies, frequently expensive cores are required and efforts are made to provide a uniform and relatively thin wall structure. An alternative to the casting of hollow bodies and solid bodies is the formation of bodies from metal foam, i.e. the metal foam casting process.
In the casting of metal foams, the skin of the casting should have a smooth outer surface while the interior structure is porous. For many purposes a metal foam casting is ideal since it is especially light weight, can provide sound damping and has a reduced thermal conductivity. It is surprising that in spite of the high porosity, metal foam castings can have high strength. However not every kind of machine part can be fabricated by foam casting.
Two basically different processes for the formation of metal foam can be distinguished, namely, one using melt metallurgical techniques and the other utilizing power metallurgy. In DE 43 26 982 C1, a typical process and apparatus for carrying out the melt metallurgical process in the production of cast bodies from metal foam have been described. A melt, for example, an aluminum melt, is held in a liquid state in two communicating vessels, one of which is provided with an agitator which generates a foam from the metal melt. The completed foam can then be forced upwardly into the mold cavity by raising the level of the liquid aluminum upon which the foam floats.
A powder metallurgical metal foam can be produced as described in DE 41 01 630 C2 from a metal powder and a foaming agent which releases gas upon heating. A mixture of the foaming agent and metal powder can be hot compacted and shaped to a blank or compact of metal particles which are held firmly together to provide a matrix in which the expanding agent particles are held in a gas-tight manner. The blanks or compacts are introduced into a heated steel mold and are foamed by heating, with the metal foam expanding to fill the mold cavity. The drawback of this system is that the contour of the blank or of the compacts must correspond to the contour of the mold cavity. Otherwise uniform foaming does not occur and frequently not all of the interstices or corners of the mold will be filled. If the blank or compact is rod or bar-shaped, it must be cut to precise lengths and positioned precisely in the mold cavity. Cold-welded locations can develop between the foaming bars which can interfere with the homogeneous distribution of the metal foam within the mold cavity.
A collapsing of pores of a foam generated by the melt metallurgical technique can be observed when the foam is pressed in the mold as is often required. Furthermore, when heated molds are used, the temperature cannot be too high or else the metal foam again will tend to collapse. In many cases, the foaming takes place in an uncontrollable manner and pores of different sizes are produced. The problem is magnified when complex shapes must be produced and thus this type of foam casting can only be used effectively for objects of simple shapes. In the melt metallurgical process, moreover, an agitator is required and positioning of the agitator frequently poses a problem since the foam is only produced in the region of the agitator and previously produced foam may tend to collapse during the agitation process. The quantity of foam produced by the agitator cannot be accurately determined so that the reproducibility of the casting process is limited. The pores of the foam generated by an agitation process do not contain gas at a superatmospheric pressure which can resist collapse and in many cases, the resulting product is inhomogeneous and nonuniform.